Background
Despite advances in breast cancer treatment, survival after recurrent metastatic breast cancer (MBC) remains poor and unevenly distributed across countries. Understanding how survival varies across comparable health systems can provide critical evidence to inform policies aimed at improving equity, access to effective therapies, and health system performance.

Objectives
To quantify international differences in survival following recurrent MBC and assess how tumour subtype, treatment patterns, and timing of recurrence contribute to observed disparities, with a focus on policy-relevant insights for equitable cancer care.

Methods
We conducted a population-based cohort study using cancer registry data from four high-income settings: Canada (British Columbia), Ireland, the Netherlands, and the United States (Connecticut). Women initially diagnosed with non-metastatic breast cancer (stages I–III) between 2005 and 2008 who developed metastatic recurrence between 2008 and 2010 were included and followed through 2015. Tumours were classified by subtype (HR+/HER2−, HER2+, HR−/HER2−). Overall survival and age-standardised net survival (ASNS) at 1, 3, and 5 years after recurrence were estimated overall, by subtype, metastatic-free interval, and recurrence period.

Results
Among 2,735 women with recurrent MBC, treatment patterns at initial diagnosis varied substantially across countries, including use of chemotherapy, radiotherapy, and endocrine therapy. Median survival after recurrence ranged from 12 months in Ireland to 18 months in the United States, with significant heterogeneity between registries. Across all countries, survival was consistently highest for HR+/HER2− disease and lowest for HR−/HER2− disease. Patients in the Netherlands demonstrated the highest net survival across most subtypes and time points; for example, 1-year ASNS for HR+/HER2− disease was 67.5% compared with 54.1% in Ireland. Survival differences persisted across metastatic-free interval strata and became more pronounced in more recent recurrence periods (2012–2014), particularly for HR+/HER2− and HER2+ disease, suggesting widening benefits from advances in systemic therapies.

Conclusions
Substantial international disparities in survival following recurrent MBC persist even among high-income countries with universal or near-universal health coverage. Differences are most evident for subtypes with effective targeted treatments, highlighting the critical role of timely access, treatment uptake, and health system capacity. These findings underscore the need for stronger science–policy integration to support equitable access to optimal therapies, improved population-level data on metastatic disease, and international collaboration to strengthen health system resilience and reduce avoidable survival gaps.